Project Summary Health information exchange (HIE) involves the electronic exchange of clinical or administrative health data across the boundaries of health care institutions, health data repositories, and states. Widespread HIE has the potential to improve health care, and HIE has been described as a critical component of recent health reform initiatives including value-based care and patient centered medical homes. Despite its potential, the evidence base for HIE is weak. Additional studies employing robust methods are necessary to strengthen the evidence base and enable examination of HIE across the myriad environments in which it is used. The purpose of the proposed study is to examine HIE use among emergency department staff using longitudinal log files from an operational HIE network involving dozens of hospitals. The specific aims focus on: 1) characterizing HIE usage patterns across multiple organizations; 2) interviewing emergency department staff about the facilitators and barriers to using HIE in the context of patient care; and 3) evaluating the relationship between HIE use and outcomes (e.g., admissions, laboratory testing) over time. Knowledge gained from the proposed study will provide evidence for or against the use of HIE. Furthermore, the research will create generalizable methods and a theoretical framework for understanding HIE use and the impact of use on health care delivery as well as clinical and population health outcomes.